A molding composition comprising a wet pulp of mineral fibers and a binder was taught in U.S. Pat. No. 1,769,519. The owner of that patent, United States Gypsum Company, has been selling a premium line of sound absorbing tiles made according to the '519 process under its ACOUSTONE trademark for more than fifty years. A rough, stone-like appearance is achieved by a casting and screeding technique. It has proven difficult to generate linear patterns on the wet pulp uniformly and reproducibly at commercially feasible costs.
The creation of linear patterns in a highly fibrous acoustical tile is often achieved by routing or sandblasting of the dry blanks. Each of these requires special equipment and expertise. Molding of the tile is conditioned upon the pulp remaining in the mold while some change, e.g. curing, drying, or setting, causes the features of the pattern to become self-sustaining.
A plastic plaster composition containing as much as 30% by weight of natural fibers is taught in U.S. Pat. No. 3,852,083 as being extrudable and moldable. Consistently good results are obtained only when a latex and a hydromodifier such as methyl cellulose are present along with the plaster and fiber. The hydromodifier enables the composition to leave the extrusion die as a smooth homogeneous column whose dimensions remain the same as the die opening. The structures obtained by the extrusion are said to be generally shape-retaining but the desirability of supporting them against deformation by gravity is also taught.
Page et al teach in U.S. Pat. No. 3,298,888 a process and apparatus for high speed, low cost manufacture of a ribbed gypsum board having paper faces. A slurry of calcined gypsum which may contain fibers is introduced between a flat bottom sheet and a pleated upper sheet in sufficient volume to fill the pleats and thereby form the ribs. The paper remains on the gypsum even after it has set, the height of the ribs having been gauged to a uniform value while the slurry has partially set but is still plastic.
It is an object of this invention, therefore, to provide a method for creating well-defined linear textures in a moving slab of wet fibrous pulp.
It is another object of this invention to provide apparatus for corrugating the surface of such a slab.
It is a further object of this invention to provide a wet pulpy slab of fibers having discrete, self-sustaining linear impressions in its surface ranging from that of a keyboard to a checkerboard to a corduroy fabric.
These and other objects are achieved by the method and apparatus described herein with reference to the drawings.